Repositioning of plasmonic hotspots along the sidewalls of conical nanoholes: a numerical investigation

Abstract

We investigate the plasmonic mode profiles along the depth of various vertical nanoholes to realize vertical nano-manipulation. Our numerical results show that by varying the incident wavelength, we can reposition the plasmonic hotspots along the sidewalls of the staircase and conical nanoholes without and with an oxide spacer. Using the Maxwell stress tensor and the three-dimensional finite difference time domain numerical method, we calculate the optical forces exerted on a polystyrene nanoparticle of 10-nm radius and the related potential energy along the sidewall, demonstrating the trapping sites corresponding to the plasmonic hotspots. Furthermore, the simulation results reveal that adding an oxide spacer of appropriate thickness adds a degree of freedom in controlling the position of the trapping site at the expense of a slight decrease in the trapping figure of merit. These results indicate that the proposed plasmonic conical nanoholes are potentially suitable for controlled vertical transportation of DNA strands.